>Bracketing seems to be the common theme, but I really don't think so.
If bracketing was switched on surely you'd have noticed the bracketing indicator on the control panel (manual p133). It sounds more like a metering issue to me - e.g. something is appearing larger at the tele end of the lens and is throwing the meter off.
I'd test the camera & 24-70 combination (on a tripod) by shooting a test chart indoors under constant artificial lighting (e.g. a tungsten spot) - see if the same thing happens.